Radio carrier telephone system



March 18, 1947. L. A. KELLEY 2,417,555

RADIO CARRIER TELEPHONE `SYMEM Filed March l, 1944 3 Sheets-Shes?I l INVEN TOR. leo/SLM@ BY v-MQ L AHORA/fr INVENTOR.

L. A. KELLEY RADIO CARRIER TELEPHONE SYSTEM 'Filed March 1, 1944siv/rcf/ March 18, 1947.

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L. A. KELLEY 'RADIO CARRIER TELEJHONE SYSTEM Filed Maren ,1, 1944 ssheets-smet sl WNS JM IN V EN TOR.

ze@ A .zfezge kmwv l /WTOR/YEY Patented Mar. 18, 1947 RADI() CARRIERTELEPHONE SYSTEM Leo A. Kelley, New York, N. Y., assigner to The BaulandCorporation, Chicago, Ill., a corporation of Illinois Application March1, 1944seria1 No. 524,819

This invention relates to new and useful improvements in radio carriertelephone systems.

The object of the invention is to provide a multiplex radiocommunication system which is simple and inexpensive to install and tooperate.

With this object in View telephone equipment at a station, e. g. a loudspeaker is so connected with a twoway channel that by the simple opera'-tion of a switch (manually or automatically operated) the station may beused to speak or to listen. A plurality of such stations and channelsmay be provided at each terminal. Preferably, one of the channelsutilizes voice current fre- 8 Claims. (Cl. Z50-9) quencies and the othervarious carrier frequencies.

The drawings diagrammatically illustrate an embodiment, Fig. 1 showingthe terminal station, Fig. 2 the significant circuits of one of thetelephone sets, and Fig. 3 a radio relay station.

Fig. 1 Vshows in diagrammatic form the electrical circuit of oneterminal of a carrier-radio telephone system. A simple system comprisestwo such terminals, alike in every respect except that the radiofrequency employed by the radio transmitter of one terminal is differentfrom that employed by the other terminal radio transmitter, the radioreceiver at each terminal being tuned to the radio frequency transmittedat the opposite terminal. Where the distance between terminals is toogreat to be spanned by such a simple system, cr where natural conditionsare such as to render the received radio signa-1 too weak forsatisfactory communication, one4 or more radio relay stations may beplaced at suitable locations intermediate the terminal stations.

One relay station comprises two radio transmitters and two radioreceivers; One radio receiver is tuned to receive the adjacent (but notat the relay station being described) radio transmitter, for example theone to the west of it, feeds its output into one of the radio relaytransmitters which transmits the received signal on a different radiofreque'ncy toward the next adjacent (but not at'the relay station beingdescribed) receiver to the east of it, in the eiiample chosen. Theremaining radio transmitter and radio receiver at the relay station areused in an exactly similar manner te accomplish relaying in the east towest direction. Carrier apparatus maybe employed at any or all oi therelay stations but the present description is not intended to coverarrangements of this sort.

Furthermore, more complicated radio networks may be considered but thisdescription is intended primarily to cover only aradio linie-'comprisingtwo terminal stations each equipped with carrier for executive use,paging, etc.

'or table, for example, near the user.

2 telephone apparatus and one or more radio relay stations not soequipped. The complete equipment at one typical terminal station willnow be described. Referring to Fig. 1, the strictly radio portion of theterminal equipment consists of a transmitter T and a receiver R, eachwith a suitable antenna. The circuit details of these units, except forthe relay TR which will be described later, are not of particularinterest since the proposed carrierradio telephone system may be used inconjunction with any type of radio telephone apparatus. The transmitterand receiver may, for example, utilize amplitude modulation or frequencymodulation, high radio frequency or low radio frequency, etc. The onlyspecial technical requirement on these units in the present instance isthat the frequency band width of the radio signal channel provided bythem shall be sucient to accommodate the total frequency band employedby the ordinary voice and carrier telephone channels connected to them.This, however, is an ordinary matter of design and is easily fulfilled.The transmitter T is provided with input terminals in and receiver Rwith output terminals out These are the terminals which are connected tothe ordinary voice and carrier telephone apparatus next to be discribed.n

As illustrated in Fig. 1, four independent telephone circuits areprovided, indicated by the four branch circuits 4converging ontransmitter T and receiver R. To put it another way, one twoway radiotelephone channel is arranged to furnish service to and from fourindependent two- Way voice and carrier telephone channels. The kactualnumber of carrier telephone channels is not important,` a greater orsmaller number being used as desired.

Attention is `now directed to the boxes designated Iv, I1, etc., attheleft-hand part of Fig. 1. These boxes are all alike, so a descriptionof one will suice. Each one, in the present system, .contains apparatussimilar to one station of an intercommunication system, a private systemof communication used in many oices and factories One of these boxes,,which will be referred to hereafter as an intercom station, is locatedconveniently on a desk This is where theuser talks and listens incarrying on a conversation.

In Fig; 1 the apparatus in thel circuit between terminals` Cv, C1,etc.,and the inand out terminals of the' radio transmitter and receiver isintended to be mounted as a unit in close prox'- imity to T and R.However, the intercom stations Iv, I1, etc., may be located where it isdesired to use them. They may be connected to terminals Cv, C1, etc., bya small gage balanced pair cable for the voice circuits and a singlewire per intercom station for energizing the associated TLR relays. In apractical case the pair of wires interconnecting an intercom I and apair of terminals C may be several thousand feet long.

Pertinent details of the intercom station are shown in Fig. 2 togetherwith the connections from it to the apparatus mounted in close proximityto the radio transmitter and receiver. The main parts comprise a voicefrequency amplifier, the box at the bottom of the figure, a, dynamictype loud speaker LS and a talk-listen switch of the well-known rotarytype.

The switch has three layers. Each layer consists of two flat arcuateconducting pieces which rotate about the center of their arcs when theswitch is actuated, and several stationary con tacts of conductingmaterial. The arcuate pieces of all three layers are rigid throughinsulating material to a common shaft which is rotated through a smallangle when the switch lever is actuated. Thus, all the arcuate piecesmove simultaneously and through the same angle or circumferentialdistance. The switch has two po sitions. As may be seen by reference tothe small diagram in the upper right-hand part of the fig- 1 When theuser desires to talk he depresses the l' switch lever to the talkposition and speaks. When he has finished speaking he releases theswitch lever which returns automatically by a spring return to thelisten position. What happens to the circuit connections will be pointedout next.

The circuit connections as shown in Fig. 2 are with the switch in thelisten position. It is seen that this connects the line terminals of theintercom station to the in terminals of the amplifier through the switchfront layer. It also connects the loud speaker LS t the out terminals ofthe amplifier through the switch middle layer. The switch rear layer inits lower part makes volume control potentiometer PI, located betweenamplifying tubes V1 and V2 active to the exclusion of P2, whilein itsupper part it keeps the winding of the TLR relay, located with thecarrier apparatus in close proximity to the radio transmitter andreceiver, de-energized by breaking the circuit to the energizingbattery. It is clear that with the switch in the listen position anyVoice currents entering on the line terminals of the intercom stationwill be amplified under the control of volume control Pl and reproducedon loud speaker LS at the desired volume. At the same time the TLR relayremains de-energized.

When the switch lever is depressed to the talk Y position, the circuitis Vas follows: The loud speaker LS is connected through the switchfront layer to the in terminals of the amplifier. The line terminals arenow connected to the out terminals of the amplifier through the switchmiddle layer. At the same time the upper part of the switch rear layercompletes the circuit for energizing the TLR relay and the lower part`makes volume control potentiometer P2` active to the exclusion of Pi.Therefore, with the switch in the talk position words spoken into theloud speaker are converted into voice currents thereby, the voicecurrents are amplied under the control ci P2, and the amplified voicecurrents at the desired amplitude are supplied to the line terminals. Atthe same time the TLR relay is energized.

It may be noted in passing that the arrangement of Fig. 2 just describeddoes away with the necessity for placing an instrument to the mouth orear as the talking and listening may be per formed some distance awayfrom the intercom station. Of course, one must be close enough tomanipulate the switch. It is contemplated that a telephone handset mightbe used which is equipped with a push button switch to connect thereceiver or transmitter portion of the handset to the voice line in muchthe same way as described for the intercom station. This push buttonswitch would also energize and de-energize the TLB. relay. It is furthercontemplated that the TLR relay may be energized by the operation of avoice operated relay well-known in the art.

The battery employed for energizing the TLR relay may take any one of anumber of forms. For instance, if the intercom station is supplied withelectrical energy from an alternating current lighting circuit, thepower transformer may have a low voltage winding connected to a fullwave rectifier of the copper oxide type and a capacity-resistance filterfor this purpose.

Returning to a consideration of Fig. l, it is apparent that the currentspassing through any pair of terminals C to or from the associatedintercom station I are ordinary voice frequency currents. The apparatusin circuit between the terminals C and the in and out terminals of theradio transmitter and receiver is a simplified car rier telephoneterminal station. It comprises one channel of ordinary voice, thecomponent parts of which are designated with a subscript v, and threechannels of superimposed carriers, the component parts of which aredesignated with a number subscript depending on the carrier `channelnumber.

The carrier telephone system illustrated and preferred is of theso-called single side band carrier suppression type. Such a system hascertain advantages which are especially desirable. In the rst place, itis economical in the frequency range required per carrier channelbecause no provision has to be made for one entire side band and a smallportion of the other. Another important advantage is that the radiotransmitter is not loaded with the carrier frequency energy which, beingan unmodulated component, does not serve to transmit intelligence in anyevent. A further advantage is the relative freedom from interferencegained by not transmitting the carrier frequency. The reason for this isthat when the carrier frequency is transmitted it is exposed toextraneous interference just as much as the side band frequencies. Sincethe carrier demodulator functions by the interaction of the carrierfrequency and side band, it follows that the ef fect of the interferenceis worse where the carrier frequency is itself affected by theinterference.

' In the carrier suppression system the carrier frequency fordemodulation is supplied locally at the carrier receiver and, therefore,not exposed to interference. Supplying the carrier frequency locally fordemodulation imposes a special requirement on the carrier frequencyoscillators in respect to frequency stability. However, oscillatorshaving the requisite frequency stability have been constructed and areentirely practical.

four channe'ls, The yordinary voice channel has no carrier6515111atp;and f cupies this frequency range from (i to 2800 Carriercljiannel Not `1 a 'carrier oscillatorfrequency offQQO `C. P. S. `andccupies a freq'ency range Dfrom sito to57oo 'crt-s. `c riet channel No.z has ro Aa carrier oscillator frequency Jof*3f .l '0() 0 P5. S. andocupies aireque'ncy range frornuGlOQ to 8700 o. i?. s. Carnet chantre; ahas a cerner "oscillator frequency of 11,90() C. P. S. and occupies afrequency range from 9 10Qto 11 7 00 C. vP. S. This recitation ofcarrier scillatorjfrequencies 'and frequency ranges occu ed,illustratesone of the adyantages of thpreferred carrierv systempreviously mentioned, namely, that it is economi-y cal rin the frequencyrange required per carrier channel. In carrier channel No. 1H, forexample, the entire upperside band, the` carrier frequency, and thatpart of the lower sideband from 5700 to 5900 C. P. S. are suppressedleaving the lower side band from 31coto'57oo o. P. s. to betransmit'ted. Upon deniodulation with the same car- 'rier frequency areprodiicedvoice range of. from 200 to 2800 C. P. Stvis secured.4 Inorder to achieve this result transmitting `both side bands,

a. frequency range from 3100 to'8700 C. P. S.

would be required, more thandouble that actually employed with thepreferred system.

Mention has already been Iiiade of the TLR relays., one of whichisiassciated with each intercorn station the sense that is, energizedwhen the switch' is in the talk position and deenergized in the listenposition. All the TLR relays .at a terminal are yconv eniently locatedwith the carrier apparatus. Referring to Fig. 2 it may be seen thateachTLR, relay'whas two armatures. The lower one has twocontacts, Aonenormally (i. e., when `de`energiaed)' closed and the other Vnmrn'allyopen'. The upper one hasfarsingle nor.. 4mally open contact. lifig. `1'shows that the lower armature connects tnejchapttel with which' it isassociated to the' output ter'jrniriialsY Voi radio re ceiver R throughits normally closedpontact when the relay is deenergized,':andy totheinput terminals of the radio' transmitter I through its normally openAContact ii/ heny tlie relay is energized. In order to" simplifythedrawing of Fig. 1, the upper armatures are not shown in proximity totheir respective energizing coils butinstea'd they appear in lagroupjustbelowthe radio transmitter T where they designated That, rrLRl", etc.,one forv each T1115?,v relay.` All' o'lj vthe armatures in this groupare Stra'lpd. together and connected to' ground. All `or their normallyopen contacts are` strappedtogether and connected toone terminal o f theenergizing coil for the transmitter relay(`4 which relay may beconveniently located iithinfthe housing oi the radio transmitter. fritecthljtehninai of the energizing coil foi theV 'IE`?." re l"a`.ya isconnected `to a' SOur Qf rllr'cifl'l@al` nsrarplfeseiied',by a battery,the othersideof which source is con nected to ground; It is evid nt4that when rthe switch ofanyonef di th u t rn-stations is' held inthe"talk" posit on, ergizing" thaassociated TLR relay, the 'IRrelay 1s alsoenergized through the closure'A ofthe? upper armatureoijthe ILR relaywith' its normallyperrlcontactgl The latter operation completes theenergizing ,circtut` ofV the TR relay'tljirdiiglrgrurclY VBeczatiisijj:of tlie'parallei connection ofthev upprarmatur tants' of the f mailreiaysgf the rn; reia'y remains feriergien when more than. one ofgtheintr om "sltiionsis .tened to the *talk* position. ioniy the,smic-nes .orali the inte'roomstatmns the listen position is the TR relayde- "eilersized- Theil?. vrelay has `one armature which coopcrates withone normally open Contact. In Fig, 1 no connections are shown for thisa'rmature and Contact as they can b ereadily describedin words.,Generallyjspeakioa thejIR relay s,des ienie.d U0 save-jon the.consumption of electrical, energy by the radio transmitter those'intermittent periods in th'ecourseo operation when allofthe channels.ab the. local terminal happen lnot to vte efrlbyed `for talkins. Radio,transliitterfr 'at the present day win employ various radio tubes .whichrequire that. .electrical ,energy be supiilied to them in order thatthey mayjfunction, Someoi this energy goes toward heating thecathodes ofthe tubes. A large partis expended in supplying the operating `voltagesfor the other electrodes, particularly the screen and plate voltages intheradio frequency power output stages. Theoretically al1 power could becut off from the transmitter whenever it is not being used at randomintervals yduring a period lof service. Practically it is often notfeasible to do this with the energy supplied for heating cathodes formany tubes used in ,transmitters because of the time lag in bringingthecathode to the operating temperature and the `danger ofharming thecathode suriaceif ahigh potential is applied to another electrode in thesame tuoe before a suitablenegative space charge has been built up inthe inter-electrodespace within the tube. However,` itis alwayspractical to cut on all power other than that required for cathodeheating during the temporary periods mentioned and efiect considerablesaving thereby, It is now clear what connections are to be made to thearmature and contact of the TR relay: they are connected in series Withthe entire source of electrical yenergy which supplies .the transmitterif that is permissible or else with that part of the source other thanthat required for heating. Elements, such as crystals, Vwhich aretemperature con.- trolled also should not be switched on and off at oddintervals during .la period oi service. The net result of the operationsof the TR,` relay is that at least a substantial part of the electricalenergyto thet'ransrnitter is disconnected whenever the TR relay isde-energized and this happens automatically every time all the localintercom' stations happen to be switched` to the listenj position'during a period of operation. 'I hus the transmitter is fully energizedonly when it is actually needed. In this way considerable electricalenergy may be saved and the life of expensive tubes extended.

lt should be noted that the functions to be performed in. the ordinaryvoice channel are fewer than those in the carrierchannels, so adescription will 4loe first given of typical operations over theordinary Voice channel. Operations over a carrier channel will then bedescribed which will be the same for all carrier channels.

rss'iirneQ` as a starting point, that the intercom stations on theordinary Voice channel at .the local and-distant terminals are standingby., Frornthe description thus far given, this means that each intercomswitch is at the listenr position and consequently that the followingsituatonexists at'eacnterniinal. The Voutput of the radiorec ev'er isconnected through the lt'mver `armature f of theTL-Ri'relay and itsnormally closcd'contact and the apparatus in the ordinary voice channelvia terminals Cv to the input terminals of the intercom amplifier. Alsothe loud speaker is connected to the output terminals of the intercomamplifier. Consequently, each intercom station Iv (one at the localterminal and one at the distant terminal) is in readiness to receive acall from the other.

Suppose the local Iv makes the oall. The person at the local Ivdepresses the switch to the talk position, makes his call by speaking tothe loud speaker and then releases the switch allowing it to return tothe listen position. Referring to the detailed description already givenof an intercom station, the speakers voice is converted into voicecurrents which are amplified the desired amount and fed to terminals Cv.From terminals Cv the voice currents now through the left-hand windingof the balanced and shielded audio transformer Tv.

Transformer Tv performs two main functions. First, because it isbalanced and shielded it preserves the balance of the circuit to itsleft despite the fact that its right-hand Winding is connected to anunbalanced circuit. Second, it provides for any desired change inimpedance level in going from one of these circuits to the other. Thesefunctions, and how they are effected in the design of the transformer,are well-known in the art.

The circuit from Iv to the left-hand terminals of Tv is preferably ofthe balanced type to secure freedom from cross talk and extraneousinterference. The circuit to the right of Tv includes the electric wavefilter LPv which, for reasons of economy in construction, is preferablyof the unbalanced type. The desirability of providing also for a changein impedance level arises from the ,fact that in many practical casesthe impedance level of the circuit to the left of Tv may be greatlydifferent from the most favorable impedance level for designing the lterLPv.

The voice currents in passing through filter LPv (in this instance fromleft to right) are transmitted only in the frequency range from to 2800C. P. S. Any voice currents having frequencies above 2800 C. P. S.entering LPv on the left are practically extinguished so far as thecurrents leaving LPv on the right are concerned. Therefore, the voicecurrents which go on to the radio transmitter T are restricted to thefrequency range 0 to 2800 C. P. S. Voice currents of frequenciesexceeding 2800 C. P- S. are not necessary in a system, such as the onebeing described, where intelligbility is the main consideration. As tothe lter LPv itself, sufiice it to say thatl it comprises a network ofinducta'nce coils and condensers and that its design and constructionare well-known in the art.

After leaving filter LPv, the voice currents are conducted to the inputin terminals of the radio transmitter T via the lower armature of theTLRv relay and its normally open contact. It will be recalled thatholding the switch at Iv in the talk position also energizes the windingof the 'ILRv relay. This explains why the right-hand terminals .of LPvare connected to the in terminals of 'I' at this time. It will also berecalled that energizing the TLR-v relay also energizes the radiotransmitterT unless it has already been energized by reason of one ormore of the otherlocal intercom stations being operated to the talkposition. Therefore, the voice currents from the ordinary voice channelnow entering the radio transmitter, find it in functioning condition.They modulate the radio frequency carrier wave in the transmitter andthe modulated radio wave is radiated from the associated antenna.

At the distant terminal the radio wave modulated by the ordinary voicechannel is intercepted by the receiving antenna and conducted to theradio receiver R. (In describing what takes place at the distantterminal reference is also directed to Fig. 1 which represents local anddistant terminals equally well.) Radio receiver R demodulates thereceived modulated radio wave and delivers at its output terminals "outa practical replica of the voice currents with which the radio Wave wasmodulated at the local radio transmitter.

Since the intercom Iv at the distant terminal is assumed to be in alistening' condition, its TLRv relay is now de-energized. As a result,the lower armature of this relay is touching its normally closed contactthereby conductively connecting the out terminals of R with theright-hand terminals of LPv. Filter LPv, just as in the case of thelocal terminal, transmits only currents in the frequency range 0 to 2800C. P. S. But it is rereiving at its right-hand terminals, from the outterminals of R, voice currents in this same range so it passes themthrough to the right-hand winding of transformer Tv.

Filter LPv transmits currents in the 0 to 2800 C. P. S. rango equallywell in either direction. At the local terminal where these particularvoice currents originated, transmission through LPv is from left toright, but here at the distant terminal it is from right to left. Thevoice currents go only where they are supposed to go by reason of thetransmission and impedance characteristics of the other electric wavefilters BF, BF2, and BFs.

From the left-hand terminals of transformer Tv the voice currents aredelivered to Iv via the terminals Cv. Since this is the Iv at thedistant terminal which is assumed to be in a listening condition, thevoice currents received by it are amplified the desired amount andsupplied to a loud speaker. Thus the person at the distant Iv hears thecall spoken by the person at the local Iv. It is clear that thedescription is the same, intfrchanging the terms local and distant, ifthe call originates at the distant Iv and that the ensuing conversationis carried on in the same manner as in making a call, the talker holdinghis switch in the talk position and the listener allowing his switch toremain in the listen position.

A description will now be given of operations over a typical carrierchannel. Only that part which is different from what has been given forthe ordinary voice channel will be covered in detail. Carrier channelNo. 1 is taken as typical. It is assumed that I, at the local terminalis about to originate a call and that I. at the distant terminal is in alistening condition. As in the case of the ordinary voice channel, theperson at the local I, holds his switch in the talk position and makeshis call by speaking toward his loud speaker. The resulting voicecurrents, amplified the desired amount, are fed to the terminals C1 ofthe No. 1 carrier channel apparatus. They then pass through the balancedlow pass lter LP1, the components of frequencies above 2800 C. P. S.being thereby suppressed. Such components are not necessary for thepractical operation of the present system, as previously explained, andif they were not suppressed at this point they would load the modulatorMD1 and the band filter EF1 with components of unnecessary frequencies.LP1 is of the balanced type becausev it is connected to balancedcircuits on both sides, i. e.y the balanced line to I1 and the balancedmodulator MB1. f

As will appear further on, in the description, MB1 functions both asmodulator and demodulator, hence the designation. At this point itfunctions as a modulator in'conjunction with carrier oscillator O1. Itcomprises four units of dry disk copper oxide rectiers, or similarrectiiiers. As shown in Fig. 1 they are connected to form a latticenetwork and are so poled as to their conducting direction that when theclosed circuit comprising the rectiers only is traced, they are allconducting in the same direction. This is not the only arrangement thatcould be used, butit is illustrated here as the preferred one.

The carrier oscillator O1 is desirably of the frequency stabilizedvacuum tube type. Suitable oscillators are well-known. In the No. lchannel O1 is set to have a frequency of 5900 C. P. S. Its outputisconnected to the midpoints of the balanced circuits to the left andright of MD1, as illustrated, and is adjusted in amplitude' so as to bemuch larger than the voice currents with which it interacts in themodulator. Stated another way, the oscillator amplitude should besufficient to produce a substantial voltage bias on the top and bottomrectifier units areconducting and the two diagonally connectedrectifiers are non-conducting. During the `next halfcycle of theoscillator, the polarity` of its output i leads is reversedcausing thetwo diagonally connected rectifiers to be conducting and the top andbottom rectiers to `be non-conducting. Thus thereotier units, under thecontrol of the carrier oscillator, act like a reversing switch on theconnections from the output of LP1 totheinput winding of T1. the voicecurrents in the left-hand winding of They reverse the directionof ow ofA itself. ls inthe ordinary voice channel it permits. the connection ,ofa balanced circuitfto an unbalanced one and effects any desired changein impedance level. of .a well-knowntype designed to transmit cur.-rents in the frequency range 3100 to 5700 C. P. S. and suppress currentsin the frequency ranges above and below these limits. Subtraction of`this frequency rangefrom the 59000. P. S. carrier shows that only thelower side band corresponding tc modulation of the carrierby voicecurrents in the range 200 to` 2.800 C. P. .S. is transmitted by. EF1.Since5900 C. P. S. is outside the transmitting range of Bliifanyresidual carrier current isxfurther reduced to negligible magnitude.

Rememberingthat the'person usingI, is holding the switch in the talkposition Whilemakng his call, the TLR1relay is thereby energized,connecting the output of the BFi filter to the in T1 at every half cycleof the carrier frequency.

It may be shown that this operation supplies the transformer T1principally with only the upper and lower side bands, sum and differencefrequencies, equivalent to modulating the carrier frequency current inamplitude by the voice currents.

Bearing in mind that this is a carrier'suppression system, it should benoted that the reversing operation by itself produces no current ofcarrier frequency in the transformer T1. However, if the oscillatoroutput is notexactly connected to the electrical midpoints of themodulator circuit, a small unbalance current of carrier frequency mayiiow in transformer T1. As is well-known, this may be made as small asdesired by improving the balance of the modulator circuit. A balancesuflicient for practical purpose may be readily obtained.

T1, therefore, passes on to theleft-hand terminals of BF; currentsconsisting of the upper and lower side bands of the 5900 C. P. S.carrier current modulatedl by the voice currents together with aresidual amount-of the carrier current terminals of. radio transmitter Tand putting it on the air'in 'case it is not already in this conditionbyreason ofi talking operations on one or more. of theA other localchannels.

Carrier. channels Nos. 2 and 3 arepreci'sely the same as carrier channelNo. 1 withthe exception of the carrier oscillator frequency andthe re.-sulting frequency range occupied by the lower sideband in each` case.Assuming that all the local intercom stations are being used fortalking, then the following situation exists. In the case of theordinary voice channel voice frequency currents in the frequency range 0to 2800 C. P, S. are supplied to the in terminals, of radio transmitterT. ,In the case of al1 the carrier channels the frequency range of thevoice currents is translated by the carrier apparatus to `occupy a rangehigher in the frequency scale than that occupied by the original voice.Thus, channel No. l supplies the` .in terminals of the radio transmitterT in the range 3100 to 5700 C. P. S.; channe'lNo. 2, 6100 to 87000. P.S.; and channel No. 3, 9l00to 11,700 C. P. S. Because each channelutilizes a 4different frequency range to actuate the radio transmitter,Vit may be operated independently of every `other channel using the sameradio transmitter. Sc far as LPv and the band filters are concerned, theindependent operation is practically secured byl designing them'totransmit in. the appropriate yfrequency ranges `with little loss, toutilize efliciently the frequency ranges between channels for changingfrom transmission to suppression so that in the frequency rangesemployed by all other channels adequate suppression is achieved. and toarrange that the impedance looking into theA right-hand terminals (asshown in Fig. l)V is high in, all `frequencyranges outside the propertransmitting range.

Continuing with the description of channel No. 1, the lower side bandcurrents (3100 to 5700 C. P, S.) resulting from the call being made at`I1 modulate, in the apparatus of the radio transemployed. l Regardlessof the particularl type of rad-io system, there will appear at the=outter- `BF1 is an electric wave filter minals vof R a practical facsimileof the lower side band (3100 to 5700 C. P. S.) currents which enteredthe in terminals of the local radio transmitter. The switch at thedistant I1 is assumed to be in the listen position and so TLRi isde-energized connecting the right-hand terminals of BF1 with the outterminals of R. Since BFi transmits currents in the frequency range 3100to 5700 C. P. S., it receives and transmits to its left-hand terminalsthe currents now coming from the out terminals of R. Selection by theproper distant carrier channel is assured because even if other distantcarrier channels happen to be connected to the out terminals of R atthis time they would reject currents in the frequency range appropriateto channel No. 1.

The lower side band currents from the lefthand terminals of BFi aretransformed by T1 which performsthe same function as at the localterminal, the only difference being in the direction of transmission.Therefore, the lower side band currents are fed to the distant MD1which, together with O1, now acts as a demodulator, As previouslyexplained, MB1 operates equally well in either direction. At the localtalking terminal it receives the voice currents to 2800 C. P. S.) fromLP1 and delivers to. T1 mainly currents of frequencies equal to 5900 C.P. S. plus 0 to 2800 C. P. S. and 5900 C. P. S. minus 0 to 2800 C; P. S.At the distant listening terminal it receives the lower side bandcurrents (3100 to 5700 C. P. S.) and delivers to LP1 mainly currents offrequencies equal to 5900 C. P. S. plus 3100 to 5700 C. P. S. and 5900C. P. S. minus 3100 to 5700 C P. S. The explanation already given forMDi acting as a modulator is the same, except for the diiferentfrequency details, as when it acts as a demodulator.

The resulting currents of frequencies 9000 to 11,600 C. P. S. togetherwith the small amount of unbalance carrier frequency (5900 C. P. S.)current are suppressed by LP1, and the resulting currents of frequencies200 to 2800 C. P. S. (facsimile of the original voice currents) aretransmitted with little loss by LP1 and delivered via terminals C1 tothe intercom station I1 at the distant terminal. Since this I1 is in alistening condition, the voice currents are amplified and applied to theloud speaker which reproduces the call made at the local I1.

This complete-s the description of the operation over carrier channelNo. 1 in one direction. It is, of course, the same in the oppositedirection, the local terminal becoming the distant terminal and Viceversa. Likewise, the operation is the same over any other-carrierchannel, the only difference being in the frequency of the carrieroscillator and the frequency range occupied by the corresponding lowerside band.

Fig. 3 shows in diagrammatic form a radio relay station which may beinterposed between the local and distant radio terminals whenever directcommunication between terminals is not feasible. The designations Westand East have no other significance than convenience in referring to thedirection of transmission. It comprises two radio receivers R1, R2 andtwo radio transmitters T1, Tz, each with associated antennas. They arepreferably the same as those used at the terminals. For west to eastoperation, Rl receives on its antenna the radio wave from the next radiotransmitter to the west, which may be either a terminal or relaystation, and produces at its out terminals currents of the samefrequencies as have already been describedfor Vthe terminal station.These currents are connected to the in terminals of the transmitter T1where they modulate a radio wave of frequencyr different fromthatreceived by the associated R1. The new modulated radio wave isradiated from the associated antenna and is picked up by the next radioreceiver (terminal or relay) to the east.

The operation from east to west is similar, the frequencies of the radiowaves being different from those employed for west to east operation.This is for the purpose of securing independent two-way radio operation.

Each radio vrelay transmitter has a TR relay which has the sameenergy-saving feature already described for the radio terminaltransmitters. The only difference is in the details of its energizingcircuit. In this respect, eachvradio relay receiver has a relay RR withone armature and its normally open contact. Relay RR1 is arranged to beenergized whenever the receiver R1 is actually receiving a radio signalof its proper frequency. A portion of this radio signal is rectified, asby radio tube VR1, and the rectified current caused to energize therelay RRl. When RR1 is energized its armature and contact close theenergizing circuit of the associated TR1 relay, thus putting theassociated transmitter T1 on the air. The overall effect is that if forany reason during a service period all the channels at a terminal happentemporarily to be in listening condition, the radio transmitter at thatterminal and all relay transmitters (if used) operating in the directionof the opposite terminal will be off the air.

What is claimed is:

l. In a carrier radio telephone system, a plurality of terminal stationseach having a radio transmitter and receiver, and a plurality oftransmission media constituting two-way channels, a communicating setfor each channel, each set having a telephone, an amplifier, and atwoposition switch connecting in one position the associated channelwith the output and the telephone with the input of the amplifier, andin the second position the telephone with the output and the channelwith the input of the amplifier, a relay for each set controlled by theassociated switch, and contacts for each relay connecting the associatedchannel in one condition of the relay to the output of the receiver andin the -other condition of the relay to the input of the transmitter atthe terminal station.

2. The system according to claim 1, and in which one channel is arrangedto transmit signals at voice frequency, and each of the other channelsat a different carrier frequency.

3. The system according to claim 1, and in which a plurality of thechannels contain each a band pass filter passing a different band offrequencies, and a low pass filter in another one of the channels.

4. In a carrier radio telephone system, a plurality of terminal stationseach having a radio transmitter and a radio receiver, and a plurality oftransmissionr media 'constituting two-way channels, each containing alow pass filter and some containing band pass filters, a communicationset for each channel, each set having a telephone, an amplifier, and atwo-position switch connecting in one position the low Apass lter of theassociated channel with the output and the telephone with the input ofthe amplier, andin the second position` the telephone with the outputand the low pass filter with the switch, and normally closed andnormally open contacts for each relay connecting the low pass filter ofone and the band pass filters of the other channels, respectively, tothe output of the receiver and the input of the transmitter at ytheterminal station.

5. In a carrier radio telephone system, a plurality of terminal stationseach having a radio transmitter and receiver, and a plurality of`transmission media constituting two-way channels arranged to beconnected in multiple to the radio transmitter and receiver, an intercomset for each channel, each set having a loud speaker, an amplifier, anda two-position switch connecting in one position the associated channelwith the output and the loud speaker with the input of the amplier, andin the second position the loud speaker with the output and the channelwith the input of the amplifier, a relay for each set energized in thesecond position of the associated switch, normally closed and normallyopen contacts for each relay connecting the associated channel,respectively, to the output of the receiver and the input of thetransmitter at the terminal station, a power control relay in eachtransmitter, and normally open contacts for the relays of all setscontrolling in multiple the power control relay.

6. In a telephone system, a terminal station, transmitting equipment andreceiving equipment at said station, a plurality of two-waycommunication lines at said station, normally open multiple connectionsbetween all said lines and the transmitting equipment, and normallyclosed multiple connections between all said lines and the receivingequipment.

14 7. The telephone system according to claimV 6, and means associatedwith each line for controlling the opening and closing of saidconnections to the line.

8. The telephone system according to claim 6, relay means associatedwith tne line controlling the opening and closing of said connections tothe line, and relay means controlling the transmitting equipmentcontrolled by the first-mentioned relay means.

LEO A. KELLEY.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,544,910 Kendall July 7, 19251,548,260 Espenschied Aug. 4, 1925 1,313,483 Heising Aug. 19, 19191,559,867 Griggs Nov. 3, 1925 1,771,700 Alexanderson July 29, 19301,814,956 Ohl July 14, 1931 1,519,626 Nichols Dec. 16, 1924 2,111,023Dixon Mar. 15, 1938 2,181,457 Peek et al. Nov. 28, 1939 2,337,878Espenschied Dec. 28, 1943 1,501,104 Espenschied July 15, 1924 1,752,346Kelley et al Apr. 1, 1920 1,690,227 Heising Nov. 6, 1928 2,289,794Martin July 14, 1942 1,709,044 Smythe Apr. 16, 1929 1,447,204Espenschied Mar. 6, 1923 2,388,906 Corderman Nov. 13, 1945

